Internally louvered sheets of the kind useful in the present invention are taught in U.S. Pat. Re. No. 27,617. These sheets or films are conveniently made by skiving a continuous web from a cylindrical billet that has been prepared by compressing an assembly of alternate circular layers of clear polymeric material and black or other opaque or transparent colored layers. To improve the clarity of the product, clear films, preferably of the same polymer as the clear layers of the billet, may be laminated or coated on each side of the skived web.
Internally louvered sheets have previously been used in combination with echelon lenses, but these previous uses have been for purposes different from purposes or objects of the present invention, and these previous uses did not involve a discovery of the improved contrast and other advantages associated with the present invention. For example, Erwin, U.S. Pat. No. 3,511,563, describes overhead projectors which use a louvered film between the stage of the projector on which transparencies bearing an image to be projected are laid and an annular fresnel condensing lens through which the projection lamp is beamed. Such a use of a louvered film is found to limit glare from the lamp that would otherwise be seen by a person using the projector, who normally stands adjacent the projector so as to be able to point to portions of the projected image, to change transparencies, etc.
In the course of the Erwin patent, it is mentioned that the louvered film and fresnel lens can be incorporated into an integral structure. However, the structures suggested in the patent are different from those of the present invention -- the structures of the patent would be large rigid structures combining concentric annular echelon lens elements and louvered film to give rise to disconcerting moire patterns --, and they represent different discoveries -- as illustrated by the fact that in the projector the louvers prevent glare to a user who is off the optical axis of the projector, while the present invention improves visibility for a viewer who is on the optical path. In the projector the louvers were a direct shield, whereas in the present invention, the louvers modify the characteristics of a lens.
Louvered films have also been used with planar transparent sheets in a display device that carries an image on the transparent sheet. In this way ambient light, either in back of, or in front of, the transparent sheet is shielded so as to darken the background, which could otherwise obliterate the image. The present invention is distinctive from such prior uses in that the structure combined with a louvered layer according to the invention has a configured surface, and the combination is found to reduce reflections that arise internally within the structure because of the configurations. A structure of the invention thus improves contrast between the image and background as transmitted by the lens, but does it without causing disconcerting moire patterns.
All in all, the present invention provides an important improvement for many uses of echelon lenses by making the images transmitted by such lenses more clear and distinctive.